A typical optical unit has been disclosed in Patent Literatures 1 and 2. The optical unit of Patent Literature 1 is mounted on a projector, and includes a digital micromirror device, a first prism, a second prism, and a third prism. The digital micromirror device is a reflective image display element, and has an image display surface with a plurality of minute micromirrors.
The first prism has a reflection surface configured to reflect illumination light, and the illumination light reflected on the reflection surface is guided to the digital micromirror device. The digital micromirror device is configured to form an image in such a manner that the illumination light is intensity-modulated by ON/OFF control of surface inclination of each micromirror. Each micromirror to be ON/OFF controlled is driven with respect to a single axis, and the inclination angle of a micromirror in an ON state and the inclination angle of a micromirror in an OFF state are different from each other.
The second prism has a total reflection surface configured to allow transmission of ON light reflected on the micromirror in the ON state and to totally reflect OFF light reflected on the micromirror in the OFF state. The ON light transmitted through the total reflection surface is input to the third prism. The third prism is configured to output the ON light toward an image projection side, the ON light having been input from the second prism. In this manner, an image is projected. In this state, the optical axis of the ON light reflected on the digital micromirror device is on the center plane of the first prism, the second prism, and the third prism in the thickness direction thereof. Moreover, the optical axis of the OFF light reflected on the digital micromirror device is on the center plane of the first prism and the second prism in the thickness direction thereof.
When the OFF light (unnecessary light) not to be used for a projected image is input to a projection lens, ghost light is caused. Moreover, when the OFF light is input to the projection lens, a local increase in the temperature of the projection lens occurs, leading to lowering of imaging performance of the projection lens. This lowers the contrast of the projected image. According to the optical unit of Patent Literature 1, the OFF light is reflected on the total reflection surface of the second prism, and is output in a direction different from that of the ON light. Thus, the OFF light is not output to the image projection side, and lowering of the contrast of the projected image due to the OFF light can be prevented.
The optical unit of Patent Literature 2 also includes a digital micromirror device for single-axis control, and a prism having a total reflection surface configured to allow transmission of ON light and to totally reflect OFF light. The OFF light reflected on the total reflection surface is input to one end surface of the prism in the thickness direction thereof, and is absorbed by a light absorption member provided in contact with the end surface. This prevents light diffusion at the end surface.
Moreover, Non Patent Literature 1 discloses a digital micromirror device (Tilt & Roll Pixel DMD) configured to drive each micromirror with respect to two axes perpendicular to each other. In this digital micromirror device, a micromirror reflects OFF light in such a direction that the optical axis of the OFF light is apart from a plane including the optical axis of ON light and the optical axis of illumination light. Thus, the digital micromirror device for two-axis control is mounted on the optical unit as described in Patent Literatures 1 and 2, and the brightness of the projected image can be improved.